Monitoring wavelength of laser devices

ABSTRACT

The invention is a circuit for monitoring the wavelength of a laser device such as a semiconductor laser. A piezoelectric resonator is positioned in the path of the light beam so as to form a Fabry-Perot cavity. The changes in the reflectivity of the cavity are detected as the resonator is driven by an oscillator circuit, and an error signal is produced in response thereto when the wavelength of the laser device strays from a desired value.

FIELD OF THE INVENTION

This invention relates to laser devices, and in particular to monitoringthe wavelength of such devices.

BACKGROUND OF THE INVENTION

In optoelectronics systems, semiconductor lasers will often experiencechanges in wavelength due to aging and temperature variations. Thisshifting of the wavelength is particularly critical in dense wavelengthdivision multiplexing applications where different channels areseparated by a small wavelength difference, typically corresponding toan optical frequency difference of 50-500 Ghz. There is often a need toclosely monitor the wavelength, and if it is drifting away from itsdesired value, to supply an error signal that is fed back to awavelength control.

A standard approach to this monitoring function would be to use twotapped optical signals passed through two optical filters and detectedby two photodetectors. The two resulting electrical signals arc passedto a balanced, differential comparator to produce the error signal. Sucha technique is suitable as long as the two optically tapped signals, theoptical filters, and the two photo-detectors all remain stable, and thisis not always the case.

SUMMARY OF THE INVENTION

The invention is a circuit for monitoring the wavelength of a laserdevice which includes a piezoelectric resonator positioned in the pathof a light beam from the device. The resonator acts like a solidFabry-Perot cavity. Means are also provided for varying the thickness ofthe resonator to change the reflectivity of the cavity as a function ofwavelength. Further means detects the changes in the reflectivity of thecavity and produces an error signal in response thereto when thewavelength of the light beam varies from a desired value.

BRIEF DESCRIPTION OF THE FIGURES

These and other features of the invention are delineated in detail inthe description to follow. In the drawings:

FIG. 1 is schematic illustration of a circuit in accordance with anembodiment of the invention;

FIGS. 2 and 3 are graphs of cavity reflectivity as a function ofwavelength illustrating a feature of the invention;

FIG. 4 is a schematic illustration of a circuit in accordance with afurther embodiment;

FIG. 5 is a graph of cavity transmission as a function of wavelength,illustrating a feature in accordance with the further embodiment;

FIG. 6 is a schematic illustration of a circuit in accordance with astill further embodiment; and

FIG. 7 is a graph of cavity reflectivities as a function of wavelengthin accordance with the embodiment of FIG. 6.

It will be appreciated that, for purposes of illustration, these figuresare not necessarily drawn to scale.

DETAILED DESCRIPTION

FIG. 1 illustrates a typical embodiment of the invention. In thisexample, a standard laser, 10, such as a 1550 nanometer, DFB single-modelaser, needs to be stabilized. The laser, 10, is biased by a standarddrive circuit, 11, to produce a light beam, illustrated by horizontalline, 12.

A piezoelectric resonator, 13, is positioned in the path of the lightbeam. The resonator includes a crystalline element, 20, such as quartz,which has a thickness, t, and is cut to vibrate in the thickness mode.Electrodes, 14 and 15, are formed on or close to both major surfaces ofthe crystal, 20. Each electrode, 14 and 15, includes an aperture, 16 and17, respectively, to permit passage of the light beam and any reflectedlight therethrough. The electrodes are driven by a standard r.f.oscillator circuit, 18, which causes the resonator to vibrate typicallyat an r.f. frequency. The r.f. oscillator, 18, may in fact be frequencycontrolled by the resonator, 13, according to the crystal's mechanicalresonant frequency. It will be appreciated that, aside from theapertures, 16 and 17, the resonator, 13, can be a standard, commerciallyavailable , thickness mode resonator.

It will be noted that a first portion of the beam, 12, illustrated byline 19, is reflected by the front surface of the crystal, 20, and asecond portion of the beam, 12, illustrated by line 21, is reflected bythe back surface of the crystal. These portions are transmitted backthrough aperture 17. The resonator, 13, is tilted slightly, (typically,the normal, 30, to the front surface makes an angle, θ, which is 1-20degrees to the beam 12), so that the reflected portions, 19 and 21(which overlap), are both incident on a standard photodetector, 22. Thephotodetector, 22, is electrically coupled to a lock-in detector, 23,which also receives, on electrical connection, 24, an r.f. signal forexample, that supplied by the oscillator circuit, 18, that drives theelectrodes, 14 and 15. The output of the lock-in detector 23 providesmonitoring of the wavelength deviation as described below. The lock-indetector, 23, is electrically coupled to the drive circuit, 11, by meansof connection 25, in order to vary the drive signal supplied to thelaser, 10, in a manner to be described.

It will be recognized that the reflected portions, 19 and 21, will forminterference patterns which are a function of the wavelength of thelaser beam, the thickness and index of refraction of the crystal, 20,and the angles of incidence which beam 12 makes with the surfaces ofresonator 20. It will be noted that the index of refraction may in turn,depend upon polarization of the beam, 12, and orientation of the opticaxis of the crystal, 20, if the crystal is birefringent. FIG. 2illustrates a portion of the reflectivity curve when no electrical driveis supplied to the resonator, where the Y-axis is the power of thecombined reflective portions, 19 and 21, as determined by thephotodetector, 22, and the X-axis is the wavelength of the laser beam,12. The values λ_(min) indicate the wavelengths at which thereflectivity is a minimum (usually essentially zero). In this example,the resonator crystal thickness, t, and tilt, θ, are chosen so that oneλ_(min) corresponds to the desired wavelength, λ_(s), of the laser, 10(i.e., λ_(min) =λ_(s)).

When the oscillator circuit, 18, applies a periodic signal (particularlynear the resonance frequency) to the resonator electrodes, the curveillustrated in FIG. 2 will shift slightly to the left and right inresponse to changes in the instantaneous thickness of the resonator 20.The lock-in detector, 23, will detect and report any changes in thesignal from the photodetector, 22, which are synchronous with the rapidvibrations of the crystal produced by the r.f. periodic signal(reference signal) applied to the electrodes of the resonator, 13, bythe oscillator circuit, 18. As long as the laser wavelength, λ_(s), isequal to λ_(min) there will be no detected change in the photodetectorsignal since the slope of the reflectivity curve at λ_(min) isessentially zero.

Assume, however, that over a period of time the wavelength, λ_(s) hasdrifted away from λ_(min) as illustrated in FIG. 3. Now, when theperiodic signal is applied by the oscillator circuit, 18, to theresonator electrodes, the lock-in detector, 23, will detect a change inthe signal from the photodetector as the curve shifts. This detectedsignal will be proportional to the slope of the reflectivity curve inthe vicinity of λ_(s) and will have a negative value in this examplesince the slope is negative. (If λ_(s) had drifted to the other side ofλ_(min), the value would have been positive.) In response to thisdetected signal, the lock-in detector transmits to the drive circuit,11, an error signal which is approximately proportional to the amount bywhich the wavelength of the laser, λ_(s) has drifted from the desiredvalue, λ_(min). The drive circuit, 11, then re-adjusts the laser biasand thereby the wavelength in order to return the wavelength of thelaser beam, 12, to its desired value. Alternatively, the error signalfrom the lock-in detector could be transmitted to means such as athermoelectric cooler (not shown), to adjust the temperature of thelaser to return to the desired wavelength. It will be appreciatedtherefore, that the primary function of the embodiment is to monitor thewavelength of the laser. The resulting error signal can be used in anumber of ways to stabilize the laser wavelength.

As known in the art, the desired thickness, t, of the resonator crystal,20, can be controlled during fabrication by monitoring the r.f.resonance frequency to achieve a wavelength, λ_(min) which is equal tothe desired laser wavelength, λ_(s). If the wavelengths do not exactlycoincide, the angle, θ, can be adjusted to achieve closer coincidence.Since quartz crystals tend to be birefringent, it is recommended thatthe laser beam have a specific polarization, which is the case for mostlaser diodes used in fiber optic telecommunications. Crystalorientations and polarization can be chosen to minimize any temperaturedependence of the resonator, 13.

FIG. 4 illustrates a further embodiment, where elements corresponding tothose of FIG. 1 are similarly numbered. In this embodiment, a portion ofthe beam, 12, from the laser is deflected by a beam splitter, 31, andthe remainder of the beam is transmitted and remains unmodulated. Theresonator, 13, is placed in the path of the deflected beam, 32, and thedetector, 22, is used to detect the portion of the deflected beam, 32,which is transmitted through the resonator rather than reflected by theresonator as in the previous embodiment. This embodiment, therefore,makes use of the transmission curve of FIG. 5, which is essentially theinverse of the reflectivity curve shown in FIG. 2, i.e., λ_(min)corresponds to maximum transmission and λ_(max) corresponds to minimumtransmission. As before, λ_(s) can be made to coincide with a λ_(min),and then when the resonator is driven, the curve of FIG. 5 will shift tothe left and right to provide an error signal if λ_(s) drifts fromλ_(min). It will be noted that, in this embodiment, the resonator, 13,can be oriented so that the normal to the surface makes an angle of zerowith respect to the beam, 32. Consequently, the invention in generalcontemplates tilt angles in the range 0-20 degrees. It will be notedthat this embodiment could be used to monitor the reflected beam, 33,from the resonator as in the embodiment of FIG. 1.

It will be appreciated that the invention utilizes a piezoelectricresonator to form a Fabry-Perot cavity where the reflectivity of thecavity as a function of wavelength is varied by driving the resonatorelectrodes. Changes in the reflectivity of the cavity are detectedeither by looking at the reflected or transmitted light, and an errorsignal is produced in response thereto when the laser wavelength driftsfrom its desired value, i.e., away from an area of essentially no slopeon the reflectivity or transmission curves.

If desired, one or more additional resonators may be placed in the pathof the laser beam, as illustrated, for example, in the embodiment ofFIG. 6. Here, the resonators 13, 43 and 53 are each driven at adifferent frequency and/or phase by oscillator circuits 18, 48 and 58respectively. The light reflected by each resonator is detected byphotodetector 22 which is coupled to a lock-in detector, 23, 43 and 53,receiving a reference signal from a corresponding oscillator circuit 18,48 and 58. Each lock-in detector produces an error signal which iscoupled to a combiner circuit, 70. As illustrated in FIG. 7, thereflectivity curves, 60, 61 and 62, of the resonators, 13, 43 and 53,will have sine waves of different frequency. The advantage of using thisdesign for a single wavelength laser is that the desired wavelengthλ_(s) can be chosen to correspond to a minimum, λ_(min), for any or allcurves as shown depending upon details in the combiner circuit, 70. Thismakes it easier to lock the laser wavelength onto the appropriateminimum value of any of the resonators. In wavelength divisionmultiplexed systems, each resonator can be used to monitor a differentwavelength. It will be appreciated that transmission mode detectionsimilar to FIG. 4 can also be used with multiple resonators.

It will be appreciated that optical coatings can be employed to alterthe reflectivity of the resonator surfaces. One can reduce the amount ofreflection and thereby preserve more of the laser beam for transmission,or alternatively, one can increase the amount of reflection of eachsurface to thereby increase the power and sensitivity of the monitoringsignal.

Various additional modifications will become apparent to those skilledin the art. For example, although in the examples given, the wavelengthλ_(s) of the laser is made to correspond to a minimum value, λ_(min) ofthe reflectivity curve, it could also be made to correspond to a maximumvalue λ_(max) (see FIG. 2). The preferred embodiment is that λ_(s)corresponds to a portion of the curve with essentially zero slope. Also,the wavelength of the laser could be monitored by use of light from thebackface of the laser rather than the forward beam 12. Further, wheremore than one resonator is employed, the resonators could have the samethickness but different tilt angles and driven by oscillator circuits ofdifferent phase or of slightly different frequencies to producedifferent, measurable reflectivity curves to aid in locking onto thedesired maximum or minimum. Alternatively, the same oscillator could beused, but a phase shift introduced before applying the r.f. signal toone of the resonators.

What is claimed is:
 1. A circuit for monitoring the wavelength of alaser device comprising:a piezoelectric resonator positioned in the pathof a light beam from the device, the resonator producing a solidFabry-Perot cavity; means for varying the thickness of the resonator tochange the reflectivity of the cavity as a function of wavelength; andmeans for detecting the changes in the reflectivity of the cavity andproducing an error signal in response thereto when the wavelength of thedevice varies from a desired value.
 2. The circuit according to claim 1wherein the resonator comprises a front and back surface, each of whichreflects a portion of the light beam to produce two reflected beamswhich form an interference pattern.
 3. The circuit according to claim 1wherein the laser device comprises a semiconductor laser.
 4. The circuitaccording to claim 2 wherein the resonator further comprises electrodeson the front and back surfaces, the electrodes each including anaperture formed therein to permit passage of the light beam through theresonator.
 5. The circuit according to claim 1 wherein the means forvarying the thickness of the resonator comprises an oscillator circuit.6. The circuit according to claim 1 wherein the means for detectingchanges and producing an error signal comprises a photodetector and alock-in detector coupled to the photodetector.
 7. The circuit accordingto claim 1 wherein the circuit further comprises a drive circuit forcontrolling the laser device, and the error signal is fed back to thedrive circuit.
 8. The circuit according to claim 1 wherein the resonatoris tilted so that a line normal to a major surface of the resonatormakes an angle within the range 0-20 degrees to the light beam.
 9. Thecircuit according to claim 1 wherein the reflectivity of the cavity as afunction of wavelength has a zero slope at a certain wavelength which isequal to the desired wavelength of the laser device.
 10. The circuitaccording to claim 9 wherein the desired wavelength is equal to aminimum on the reflectivity curve.
 11. The circuit according to claim 6wherein the photodetector is positioned to receive a portion of the beamreflected by the resonator.
 12. The circuit according to claim 6 whereinthe photodetector is positioned to receive a portion of the beamtransmitted through the resonator.